Work machines such as, for example, excavators, loaders, dozers, motor graders, and other types of heavy machinery use multiple actuators supplied with hydraulic fluid from a common pump on the work machine to accomplish a variety of tasks. These actuators are typically velocity controlled based on an actuation position of one or more operator interface devices. For example, an operator interface device such as a joystick, a pedal, or any other suitable operator interface device may be movable to generate a signal indicative of a desired velocity of an associated hydraulic actuator. When an operator moves the interface device, the operator expects the hydraulic actuator to move at a corresponding predetermined velocity. However, when the movement of one of the actuators is restricted by an external load, the pressure of the fluid moving the actuator may rise abruptly. Similarly, when movement of an actuator associated with a heavy work implement is initiated, the pressure of the fluid moving the actuator may also rise abruptly due to inertia of the heavy work implement. These abrupt rises in pressure may reduce fine controllability of the actuators and produce actuator velocities that are unexpected and/or undesired. In addition, because the pressure of the fluid supplied to all of the actuators is controlled by the single highest pressure of any one actuator, during these situations of abrupt pressure rises, the fluid supply to and the velocity of all of the actuators will be reduced.
One method of improving controllability and predictability of actuator movement is described in U.S. Pat. No. 4,938,023 (the '023 patent) issued to Yoshino on Jul. 3, 1990. The '023 patent describes a fluid pressure control system for a hydraulic excavator. The fluid pressure control system has a first selector valve for controlling the movement of a first actuator, and a second selector valve for controlling the movement of a second actuator. The first and second selector valves are pilot operated. A variable relief valve is fluidly connected between the first actuator, the first selector valve, and a tank to relieve and thereby vary the pressure of the fluid supplied to the first actuator in response to the pilot pressure supplied to the first selector valve. For example, if the pilot pressure supplied to the first selector valve is low, the pressure relief setting associated with the first actuator is low. Likewise, if the pilot pressure supplied to the first selector valve is high, the pressure relief setting associated with the first actuator is high. This variable relief setting functions to relate the magnitude of actuator pressure to pilot pressure directly controlled by the operator, thereby giving the operator more control over movement of the first actuator and associated heavy or movement-restricted work tool.
In addition, the fluid pressure control system of the '023 patent may include a third selector valve operated by the same pilot fluid that operates the second selector valve. The third selector valve may function to block feedback from the first (high inertia) actuator to a common supply pump when the second actuator is operated. In this manner, an abrupt rise in pressure associated with the first actuator will not reduce the flow of pressurized fluid to the second actuator and the resulting velocity of the second actuator.
Although the fluid pressure control system of the '023 patent may improve controllability and predictability of fluid actuator velocity, it may be inefficient and limited. In particular, because the fluid pressure control system of the '023 patent improves controllability by relieving pressurized fluid to a tank, the work associated with pressurizing the relieved fluid may be lost, thereby reducing the efficiency of the hydraulic excavator. Further, because any operation of the second actuator, regardless of magnitude, blocks feedback from the first actuator to the common pump, the pump of the '023 patent may be inefficiently operated during minor operations of the second actuator. In addition, because the fluid pressure control system of the '023 patent is purely hydro-mechanical, tunability of the system may be limited.
The disclosed control system is directed to overcoming one or more of the problems set forth above.